A security plug (10) adapted to be inserted into the female socket (18) of a fastener (20) to prevent a tool, such as an Allen key, engaging in the female socket. The plug (10) has an outer surface (22) complementary to the female socket (18) and a retaining member (14) that retains the plug (10) in the female socket (18), and at least one removal portion (26, 70) adapted to be engaged by a second tool (100), whereby engagement of the second tool (100) and the plug (10) allows forces applied by the second tool (100) to the plug (10) to withdraw the plug from the female socket against the action of the retaining member. The second tool and the removal portion (26, 70) may have complementary Morse tapers and the retaining member (14) may be a magnet.